The FDA’s Office of Combination Products would develop regulations and guidance for any combination products that are presented by a company. The Office of Combination Products would review and assign responsibility for combination products to one of the branches of the FDA, which are CDRH, CDER, and CBER. Its assignment to which branch of the FDA will come down to the primary mode of action (PMOA). Once assigned it will be subjected to its assigned branch’s classifications and regulations. So for your mentioned drug delivery system I would think its PMOA would be the drug component since that is what the most important aspect of it is so it would be assigned to CDER and for the implant treated with an antibiotic, I think the primary focus would be the implant characteristic so it would be assigned to CDRH.

Link:
https://www.fda.gov/CombinationProducts/RFDProcess/default.htm

There have are many instances when the device has dual capabilities which falls in both the CDRH and CDER but first we have to determine what the primary purpose for device for example its a antibiotic releasing hip stem than it primary purpose is to support hip rather than drug release so it first falls in CDRH class 2 medical device than we have to make sure it also complies to CDER for it's antibiotic part. In this case it was simple to identify and classify your device. What if you cant classify your device Request of Designation (RFD) can be filled in which case FDA can identify for you and than you can adhere to all rules accordingly.

I think that many devices can fall between multiple sectors. The way I look at it to understand why certain devices go to the Center for Devices and Radiological Health, the Center for Drug Evaluation and Research, or the Center for Biologics Evaluation and Research is to take away one aspect of the device and then the other to see which is the primary mode of action. For the example you gave, a device that slowly releases a specific drug at a controlled rate, although the controlled releasing rate is an important factor if the drug was not present then the releasing device is pointless. If the drug was taken out of the picture then there would be nothing for the device to release.

I agree with @wms7 that the Office of Combination Products (OCP) would review the primary mode of action (PMOA) to determine the agency assigned; CDRH, CDER, and CBER.

When looking at the additional OCP link @wms7 provided, I found that if OCP doesn't have enough information to determine the PMOA of a product directly or cannot determine the PMOA with reasonable certainty, OCP will assign the combination product in accordance with the assignment algorithm at 21 CFR 3.4(b).

So What is the Assignment Algorithm? The Assignment Algorithm has 2 steps:

Step 1. If there are other combination products that present similar questions of safety and effectiveness with regard to the combination product as a whole, the Agency will assign the combination product to the Agency component that regulates those other combination products. See 21 CFR 3.4(b).

Step 2. Assign the combination product to the Agency component with the most expertise related to the most significant safety and effectiveness questions presented by the combination product. This would be used in cases for example, the RFD might present the first such combination product, or the combination product presented might differ in its intended use, design, formulation, such that it presents different safety and effectiveness questions than existing combination products.

Overall, the FDA gives clear guidance on how to write a Request for Designation and the FDA will provide a designation letter within 60 days.

https://www.fda.gov/RegulatoryInformation/Guidances/ucm126053.htm#IIIA

In the case of an implant coated with antibiotics, I would guess that the FDA would designate the product as a device because its primary mode of action would be as an implant, not as a antibiotic. The reason being that the primary therapeutic aspect of this product would be to serve as an implant. It would be up to the FDA's Office of Combination products to designate any device where the primary mode of action is not certain. If the the FDA does end up classifying this product as a device it would fall under the regulation of CDRH.
In the example of the drug delivery system I would guess that the FDA would designate that as a drug because it primary therapeutic function is the release of the drug. If classified as a drug then the drug would fall under the regulation of CDER.

Continuing from alexandrabuga's response, a Request for Designation is a written to the Office of Combination Products (OCP) requesting regulatory classification to determine if it is a device, drug, and/or biologic product and which Agency will be assigned. The FDA formally responds to RFD's by returning a letter of designation. The letter of designation is a binding commitment to the device classification and it's assigned agency. These terms may only be subjected to change only under the conditions detailed in Section 563 of the FD&C Act and 21 CFR 3.9. A change in classification or the product's assigned agency typically occurs when the primary mode of action (PMOA) differs from what was originally described in the RFD.
http://www.fda.gov/CombinationProducts/GuidanceRegulatoryInformation/ucm109108.htm

Now a days many companies have been trying to release a multi purpose device with combination of drug and device. In this case products are classified according to nature of primary job it was originally made for but in cases when it is difficult to identify which organisation does it fall under a written request is send to FDA it is called IDE in which FDA tells which class under which it would fall under but all this is time consuming but on a second thought this time is worth spending rather than getting it wrong.

From my work experience with Panasonic. I had a chance to work a 3D Anatomy project. Th main purpose of the project is education. It is a simulation software for the anatomy of the body and allows you to dive in different layers. I think this type of project classifies as class 1. however not sure which department is responsible for such a project, is it CDRH? Because it is learning software and not really a medical device. you can search the product on Google. Also, there is simulation videos on youtube.

The Food and Drug Administration (FDA) is amending its combination product regulations to define “mode of action” (MOA) and “primary mode of action” (PMOA).
Where,
MOA is the means by which a therapeutic product achieves an intended effect or action.
and PMOA is the single mode of action of a combination product that provides the most important therapeutic action of the product. The most important therapeutic action is the mode of action expected to make the greatest contribution to the overall intended therapeutic effects of the combination product.

When the agency cannot determine with reasonable certainty which mode of action provides the most important therapeutic action of the combination product, the final rule sets forth an algorithm the agency will use to assign combination products to an agency component for regulatory oversight.

Lastly, the final rule will require a sponsor to base its recommendation of the agency component with primary jurisdiction for regulatory oversight of its combination product by using the PMOA .The final rule is intended to promote the public health by codifying the agency's criteria for the assignment of combination products in transparent, consistent, and predictable terms.

FDA legal authority to regulate both medical devices and electronic radiation- emitting products in the federal Food Drug& Cosmetic Act. To fulfill the provision of the FD&C Act that apply to medical devices and radiation - emitting products, FDA develops, publishes and implements regulations. The FDA regulates several industries. Medical devices. Food and beverages. Drugs, Cosmetics, and each industry has its own regulations.

Recently there are more combination products arriving in the market with dual capabilities falling in CDRH and CDER. In this case the products are classified based on the primary work of action; and if the device is not classified then Request of Designation (RFD) should be filled and send to FDA for classification of the device. The FDA regulates and implements the rules and regulation.

After listening to the lecture this week one statement stuck with me. Dr. Simon was talking about a bone plate and he was saying if you claim that the bone plate can cure cancer that would make it a class 3 device even though we know a bone plate would be a class 2. As written in the powerpoint "Product claims determine pathway". For combination devices I believe it would matter on what the product or device is meant for. For example for a device that slowly releases a drug the FDA would regulate as a drug because that is what the device is meant for. The main function of the device is to release the drug into the body. For an implant with antibiotics on the surface would be different in my opinion. For example a hip implant is meant to "replace" a hip and help a person gain better motion and movement by replacing their hip. But this is still very iffy as well because antibiotics should not be put in a patient without being tested. This could go either way I think. 

With the different regulation requirements for medical devices and drugs individually, I do believe that when a combination of the two is at play, that both regulation requirements need to be done to have the product be approved by the FDA. It would get complicated with the process because of all the different clinical trials needed an implant as well as the drug regulations. There would need to be a RFD for the designation of the primary mode of action of the product. Dependent on which primary mode of action is decided, there are steps to follow. It could be CDER, CBER, or CDRH. In this case, since a drug is involved, there would be more steps involved with clinical trials and documentation for the addition of the drugs involvement.

If a product overlaps between two or more kinds of controlled or regulated products, they will need regulatory clearances from both or all of the agencies in charge of that. To get clear directions of which agencies to go to, the FDA can point people in the right directions to go get their products cleared. Every aspect of a product has to be cleared so that any member of the public cannot get hurt by anything undisclosed. This protects us as the public, and us as the engineers/companies.